Either cooler will be good.
Also look at the Xigmatek dark night S1283v which has had some good reviews and is cheaper:http://www.newegg.com/Product/Product.aspx?Item=N82E16835233029
If the 4850 suits you, why not keep it?
The next step up would be GTX260-216 or 4870
Another step would be a GTX275